Scale deposition is a common problem encountered in oilfield operations. Scaling can be caused by a temperature or pressure change, out-gassing, a pH shift, or mixing of incompatible waters. The deposited scale adheres on the surfaces of perforations, casing, production tubing, valves, pumps, and downhole equipment, thereby clogging the wellbore and interfering with fluid flow. Scale deposition can also harbor bacteria and facilitate corrosion.
Scale inhibitors have been used in oilfield applications to minimize or prevent scale deposition. Scale inhibitors can be deployed downhole by squeeze treatment. Additionally, scale inhibitors can be added downhole by injection through capillary strings or via the annulus and injection ports on topside processing equipment. Once introduced downhole, the scale inhibitors are adsorbed onto the rock formation and slowly released back into the water over time. Scale inhibitors prevent the nucleation of deposits and/or impede crystal growth after nucleation has occurred. Occasionally scale inhibitors can act as dispersants.
To effectively control scale deposition, the scale inhibitors have to be present above a certain concentration. The minimum inhibitor level required to prevent scale deposition is commonly referred to as “minimum inhibitory concentration” (MIC) or “minimum effective concentration” (MEC).
During oil and gas production, scale inhibitor is injected into the reservoir and allowed to return gradually. The concentration of scale inhibitors will decrease over time until such time that the concentration is below the MIC or MEC level. Once the concentration of scale inhibitors falls below the MIC or MEC level, the scale inhibitors can no longer effectively prevent scale formation. Thus, monitoring scale inhibitor residual levels from produced water is a proactive way to determine the need for additional scale inhibitor treatments or the need for changing treatment levels.
Various techniques exist for determining the level of scale inhibitors in aqueous systems. Existing methods include the Hyamine® precipitation method, the colorimetric method, and elemental analysis techniques, such as inductively coupled plasma, atomic absorption spectroscopy, atomic-emission spectroscopy, or high performance liquid chromatography.
However, multiple analytical techniques are typically needed in order to analyze the entire set of anionic scale inhibitors. In addition, analyses are often complicated by factors such as dissolved ions (i.e., chloride), high salinity, and other matrix effects such as dissolved organic compounds. The existing methods can further have one or more disadvantages including labor intensive sample preparation and analysis, long turnaround times, a requirement for excessive replicate instrumentation, and expensive and complex analytical instrumentation.
Thus, the art would be receptive to improved methods of determining the level of scale inhibitors in water systems. Advantageously, the improved methods are robust, high throughput methods capable of analyzing different classes of scale inhibitors without the need for de-salting, sample cleanup, extensive sample handling and high capital equipment investments.